Method of and means for controlling high-frequency alternating currents



Q Voltage 0f Genefator or- Amplifier W/bd/hg E. F. W. ALEXANDERSON.

METHOD OF AND MEANS FOR CONTROLLING HIGH FREQUENCY ALTERNATING CURRENTS.

APPLICATION FILED JAN. 21, 1916.

1,328,610. I Patented Jan. 20, 1920.

AAAAAAA "Iv" Inventor 1 His Attornel UNITED STATES PATENT OFFICE.

ERNST F. W. ALEXANDERSON, OF SCI-IENECTADY, NEW YORK, ASSIGNOR TOGENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

METHOD OF AND MEANS FOR CONTROLLING HIGH-FREQUENCY ALTERNATING ICURREN'IS.

Specification of Letters Patent.

Patented Jan. 20, 1920.

Application filed January an, 1910. Serial No. 73,443.

To all whom it may concern:

Be it known that 1. Elmer F. Vt. ALFX- ANDERSON, a citizen of the UnitedStates, residing at Schenectadv. Schenectady, State of New York, haveinvented certain new and useful Improvements in Methods of and Means forControllin Hi h-Frequency Alternating Currents, 0 which the following isa specification.

My present invention relates to improvements in apparatus forcontrolling alternating electric currents, and more especially forcontrolling alternating currents used in wireless signaling systems.

The object of my invention is to provide means for efficientlycontrolling and varying the amplitude of a high frequency currentflowing in a signaling circuit in accordance with variations'in theamplitude of a signaling current of much lower frequency and amplitude.

In carrying my invention into effect 1 make use of electromagneticapparatus of a kind which I term magnetic amplifiers, such, for example,as are described in my U. S. Patent No. 1,206,643. By varying thecircuit arrangements used inadapting a magnetic amplifier to asignalingsystem over those which have heretofore been employed, and by using inconjunction therewith suitable condensers in a way which will be morefully set forth in the description which follows, I am able toaccomplish the object of my invention.

The novel features which I believe to be characteristic of my inventionare pointed out with particularity in the appended claims. The inventionitself, however, with further objects and advantages will best beunderstood by reference to the following description taken in connectionwith the accompanying drawing in which Figure 1 is a simple diagrammaticrepresentation of the apparatus and circuit connections which may beemployed in carrying my invention into effect; Fig. 2 showsdiagrammatically the preferred form of magnetic amplifier used with thearrangement of the magnetizing winding thereon; Fig. 3 is agraphicrepresentation of the volt ampere characteristics of the amplifier underdifferent conditions" of operation; Fig. 4 shows the combined voltampere characteristics of a high frequency inthe county of alternatorand of a magnetic amplifier with different values of control current;Fig. 5 is a diagrannnatic representation of a modification in theapparatus and circuit connections which may be used and Fig. 6 is adiagram showing how the system of Fig. 1 Inlay be modified so that itmay be used for to egraphy instead of telephony.

As indicated in Fig. 1, the armature wind ing 1 of a high frequencyalternator, the rotor 2 of which is driven by a motor 3, is inductivelyconnected by means of the transformer A with the antenna 5. Coils 6 and7 of a magnetic amplifier are connected in shunt to the secondary oftransformer 4 and in parallel to each other. A variable condenser 8 isconnected in series with both coils and condensers 9 and 10 areconnected in series with coils 6 and 7 respectively. A condenser 11 mayalso be connected in shunt to the two coils. By varying the currentflowing through coils 6 and 7, the electromotive force at the terminalsof the secondary of transformer 4 may be varied and the amplitude of thecurrent flowing in the antenna will be varied accordingly. The means forvarying the current flow in coils 6 and comprises two controlling coils12 and 13. Coil 12 is supplied with a constant direct current from thesource 14 and the value of this current required to give the magneticcircuit of the amplifier a desired degree of saturation may be adjustedby the variable resistance 15. The winding 13 is supplied with thesignaling-current which in the present example is furnished by thesecondary of transformer 16, the primary circuit of which includes atelephone trans mitter 17 with the usual local battery 18. A highinductance 19 is preferably inserted in the circuit of coil 12 in orderthat this circuit shall offer a. high impedance to the flow of currentsinduced therein from the coil 13. It is also desirable that the circuitof coil 13 shall be resonant to the most common frequency of the currentin that circuit and for this purpose a variable condenser 20 in serieswith the circuit may be employed. This condenser will preferably beshunted by resistance 21 in order that the circuit shall not be toohighly resonant.

By the organization which I have thus described it is possible by meansof the small methods.

'an amplification of the current variations in the telephone transmitterhaving. a ratio equal to the ratio between the frequency of the highfrequency current flowing in coils 6 and 7 and that of the current whichis to be amplified. While the systei'n here shown is especially adaptedfor wireless telephony,

it may, by suitable imidification, equally well be employed fortransmitting telegraphic signals.

()ne manner in which this maybe accomplished is indicated in Fig. (l inwhich the coil 13 is included in the circuit with a telegraph key 31 andbattery 32.

In Fig. 2, I have shown a magnetic structure which is especiallysuitable for a magnetic amplifier. In this case two magnetic cores 22and 23 carry the coils 6 and 7. The windings are so arranged that theflu produced by the high frequency current in the coils is in oppositedirections in the two cores and as a result this flux is confined to thetwo cores and the small portions 24 of the structure adjacent the cores.The winding 25 which carries the controlling current is wound aroundboth of the cores as indicated. Theflux produced by this windingtraverses the two cores in the same direction and also traverses theouter legs 26 of the magnetic structure. It will be apparent that eachturn of the controlling winding is threaded by the flux 1n the cores 22and 28 in opposite dlrections so that any electrometive forces inducedin the winding bythe high frequency flux are neutralized in each turn.While I have shown a single controlling winding this may be divided intotwo separate parts as indicated in Flg. 1.

Magnetic amplifiers of the type which I employ may be operated by twodistinct In case the-two high frequency coils are connected in series,as shown in my U. S. Patent #1,206,643, the current in those coils isforced to be equal at any moment and the resulting flux is allowed toadjust itself in accordance with the permeability of the iron so thatthe Voltage appearing at the terminals of the high frequency wlndingbecomes a variable function depending upon the current distribution. Incase the two coils are connected in multiple as I have indicated in Fig.1, the relation of the currents in the two coils is indefinite but theflux variation in the two branches of the core is forced to be equal. InFig. 3, curve A shows the volt ampere characteristic of the coils withthe. first method of opmeans greater sensitiveness and a lower curvaturemeans that larger currents can be carried without causing instability,as will be explained later.

With the multiple connection the flux variations are forced by thecirculating current flowin in the short-circuit between the two multip ecoils. The induced current in this short circuit tends to oppose anychanges in the average flux and thus a tele phone current in the controlwinding would simply cause a corresponding short circuit between the twohigh frequency coils withoutproducing the desired flux variations. Thisdifficulty, however. can be overcome by taking advantage of the factthat the high frequency winding needs to operate only at radiofrequencies which are very much higher than the frequency of thetelephone current. It is, therefore, possible to find a value for thecondensers 9 and 10, such that this circuit acts as a short circuit forthe radio currents and ractically as an open circuit for the telep onecurrent.

The current in' the hi h frequency winding has a tendency to (epress theaverage average flux of the high frequency coil although the controlexcitation is kept constant. It has also been shown above that thedegree to which the short circuit opposes changes in the average fluxcan be modified by the introduction of suitable condenseis.

In order to show how by other modifications the magnetic amplifier maybe utilized to advantage for controlling the voltage of an alternator,reference may be had to the alternator characteristics shown in Fig. 4.The alternator voltage is plotted against the current in the shuntcircuit and curves G, G and G" represent the alternator characteristic.for different outputs. The magnetic amplifier is used in the controlshunt circuit and the volt ampere characteristics of the i citation inthe controlling winding. The

intersections between the sets of curves give the alternator voltages atthe corresponding amplifier excitations.

These curves show that the alternator voltage approaches zero as theamplifier excitation increases and it is possible to'obtain effectivecontrol of telegraphy in this way without exceeding the current carryingcapacity of the control winding of the amplifier. However, these resultsare'not all that'is to be desired for telephony because there is a lackof linear proportionality between the alternator voltage and amplifiercontrol current. The improvements in the characteristics of theamplifier that are described have the eifect not only of securing thislinear proportionality but also a much higher degree of sensitiveness.The nature of these improvements can be explained by further referenceto the volt ampere characteristics. These characteristics are a set ofcurves radiating from the origin, each of which represents a certaincontrol excitation. For the complete control. of the alternator, to varyits voltage from zero to a'maximum, it would be desirable that the firstcurve should coin: cide with the Y-axisand the last coincide with theX-axis, and that the other curves should be distributed. uniformlybetween these limits. Instead of this, however, the amplifiercharacteristics all fall within a certain angle covering only a littlemore than one-half the desired sweep. This disadvantage may be overcometo a large ex tent by neutralizing part of the voltage of the amplifierwindmg by a suitable series condenser 8 so that the circuit becomesresonant when the radiation from the antenna is a minimum andneutralizing part of the current by a suitable shunt condenser 11 sothat the circuit which includes the coils and the shunt condenser becomeresonant when the radiation from the antenna is a maximum. If thecondenser is chosen so that it exactl neutralizes the inductance of theampli er winding at some definite value of excitation, the resultingimpedance at this excitation becomes a m1nimum and the impedance at anylower excitation is determined by'the difference between the inductivereactance of the amplifier coil and the capacity reactance of the seriescondenser. The smaller this difference, the lower will be the amplifierexcitation. which gives minimum impedance vand the correspondingminimum. of alternator voltage. This means that the sensitiveness of theamplifier I is increased because a smaller excitation is needed toreduce the alternator voltage. The increase of sensitiveness which canbe obtained in this way is, however not unlimited. If the minimumimpedance is obtained as a result of a large inductive and a largecapacity reactance, the core in my prior application, Serial No.873,110,

filed November 20, 1914.

The voltage which results from the combination alternator and amplifiercan be determined by the intersection of the alternator and amplifiercharacteristics. When these curves have a definite sharpintersectionpoint, a definite alternator Voltage results from each excitation of theamplifier. If on the other hand the curves have such a shape that thealternator and amplifier characteristic curves become parallel ortangent in some place, the intersection becomes indefinite and theresult is instability and generation of self-excited oscillations. Theconditions which lead to the instability may be illustrated by thecurves-shown in broken lines in Fig. 4. If a series condenser is usedwhich has such a value that it has the voltage characteristic indicatedby the curve C, the resultant characteristic of the shunt circuit fordifferent values of excitation will be represented by the difierencebetween this curve and the curves H, H, etc. For example, curve H willbe brought down so that it occupies a position H and curve H"" will bepractically coincident with the X-axis. All of the new characteristicswill have definite intersection points with the generatorcharacteristics and the operation will, therefore, be stable under allconditions of load which are indicated. If, however, a condenser is usedwhich is of such capacity that it has the voltage characteristicindicated by the curve C, the resultant of curve H wlth this condensercurve will be the curve H It will be seen that this curve becomes tanent to the generator characteristic G and, t erefore, operation withthis generator output and this particular value of excitation will beunstable. With higher degrees of excitation, it will be apparent thatthe point of instability will be reached with lower generator outputs. f

In Fig. 5 I have shown an arrangement which is the equivalent of thatshown in Fig. 1, only in this case a separate winding 27 on thehighfrequency alternator supplies current for the operation of a secondamplifier 28. The circuit through the coils of this amplifier iscontrolled in the same manner as the current in the amplifier coils inthe organization shown in Fig. 1. The variable indicated but onecontrolling winding in dicated in the supply connections for thecontrolling winding 31 of amplifier 28.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. The combination in a wireless signaling system of an antenna, asource of high.

frequency alternating current) connected to said antenna, anelectromagnetic device comprising a pair of wmdmgs connected m shunt toeach other and included in a circuit with said high frequency source, apair I of parallel cores magnetized by said windings in such a way thatthe flux produced therein flows in opposite directions in said cores, acontrolling winding surrounding said high frequency windings so as toproduce a flux in the same direction through both of said cores, meansfol-supplying said controlling winding with a variable current, thevariations of which are of comparatively low frequency in com arisonwith the frequency of the current in the high frequency circuit, acondenser in series with said high frequency winding. to neutralize partof the voltage of said windings, a condenser in shunt with said highfrequency windings for neutralizing a part of the'current therein, andcondensers in the closed circuit formed by the high frequency windingsof such capacity that the circuit offers a' low impedance to highfrequency currents but.

a high impedance to currents of the frequency of the controllingcurrent.

2. The combination in a wireless signaling system of an antenna, asource of high frequency alternating current connected to said antenna,an electromagnetic devicecomprising a pair ofwindings connected in shuntto each other and included in a circuit with said high frequency source,a air of parallel cores magnetized by said windings in such a way thatthe flux produced therein flows in opposite directions in said cores, acontrolling winding surrounding said high frequency windings so as toroduce a flux in the same direction throug both of said cores, means forsupplying said controlling winding with a variable current, thevariations of which are. of comparatively low frequency in comparisonwith the frequency of the current in the high frequency circuit, acondenser in series with said high frequency windin s to neutralize partof the voltage of sai windings and a condenser in shunt to said highfrequency windings for neutralizing part of the current therein.

3. The combination in a wireless signaling system of an antenna, asource of high frequency alternating current connected to with said highfrequency source, a

said antenna, an electromagnetic device comprising a pair of windingsconnected in shunt to each other and included in a circuit air ofparallel cores magnetized by said windings in such a way that the fluxproduced therein flows in opposite directions in said'cores, acontrolling winding surrounding said high frequency windings so as toroduce a flux in the same direction .throug both of said cores, meansfor supplying said controlling windin with a varlable current, thevariations 0 which are of comparatively low frequency in comparison Witthe frequency of the current in the high frequency circuit, a condenserin series with said high frequency windings to neutralizepart of thevoltage of said windings and condensers in aclosed I circuit formedbythe high frequency windings of'such capacity that thecircult offers alow impedance to high frequency currents but a high impedance tocurrents of the frequency of the controlling current.

4. The combination in a wireless signaling system of an antenna, asource of high frequency alternatin current connected to said antenna,an e ectromagnetic device comprising a pair of windings connected inshunt to each other and included in a circuit with said highfrequency'source, a pair of parallel cores magnetized by said windingsin such'a way that the flux produced therein flows in oppositedirections in said cores, acontrolling winding surrounding said highfrequency windings so as to produce a flux in the same direction throughboth of said cores, means for supplying said controlling windin with avarlable current, the varlations'o which are of comparatively lowfrequency in comparison with the frequency of the current in the highfrequency circuit, a condenser in shunt to said high frequency windingsfor neutralizing a and condensers In a closed circuit formed by the highfrequency windin of such capacity that the circuit offers a ow impedanceto high frequency currents and a high impedance to currents of thefrequency of the controlling current. I I

5. The combination in a wireless signaling system of an antenna, asource of high frequency alternating current connected to said antenna,an electromagnetic device comprising a pair of windings connected inshunt to each other and included in' a circuit with said high frequencysource, a pair part of the current therein,

rent, the variations of which are of comparatively low frequency incomparison with the frequency of the current in the high frequencycircuit, and a condenser in series with sand high frequency winding'toneutralize part of the voltage of said wind- 1ngs.-

6. The combination in a wireless-signaling system of an antenna, asource of high frequency alternating current connected to said antenna,an electromagnetic device comprising a pair of windings connected inshunt to each other and included in a circuit with said high frequencysource, a pair of parallel cores magnetized b said Wllldings in such a,way that the ux produced therein flows in opposite directions in saidcores, a controlling winding surrounding said high frequency windings soas to produce a flux in the same direction through both of said cores,means for supplying said controllin windin with a variable current, thevarlations 0 which are of comparatively low frequency in comparison withthe frequency of the current in the high frequency circuit, and acondenser in shunt with said high frequency windings for neutralizing apart of the: current therein.

7. The combination in a wireless si aling system of an antenna, a sourceof igh frequency alternating current connected to said antenna, anelectromagetic device comprising a pair of windings connected in shuntto each other and included in a circuit with said high frequency source,a pair of parallel cores magnetized by said windings in such a way thatthe flux-produced therein flows in opposite directions in said cores, acontrolling winding surrounding said high frequency windings so as toproduce a flux in the same direction through both of said cores, meansfor supplying sald controlling winding with a variable current, thevariations of which are of comparatively low frequency in comparisonwith the frequency of the current in the high frequency circuit, andcondensers in a closed circuit formed by the high frequency windings ofsuch capacity that the circuit offers a low impedance to high frequencycurrents but a high impedance to currents of the frequency of thecontrolling current.

8. The combination in a wireless signal-" ing system of an antenna, asource of high frequency alternating current connected to saidantenna,an electromagnetic device corn-- prising windings included in shunt toeach other in a circuit with said high frequency source and a condenserin series with said windings'to neutralize of said windings. s

9. The combination in a wireless signaling system of an antenna circuit,a source with sai part of the voltage comprising a pair of windingsincluded in shunt to each other in a circuit with said high fr'eguencysource, a condenser in series frequency windings, for neutralizing partof the current therein.

high frequency windings to neu-.

11. An elctromagnetic device for controlling the flow in an electriccircuit of high frequency current comprisng a pair of windings connectedin shunt to each other and included in the high frequency circuit,

a condenser in series with said high frequency windings to neutralizepart of the voltage of said windings, and a condenserin shunt with saidhigh frequency windings for neutralizing part of the current therein.

12. Means for controlling the flow of high frequency current in anelectric circuit complying said controlling winding with the controllingcurrent and a condenser in series with said high frequency windings toneutralize part of-the voltage of said windings.

. l3. Means for amplifying a variable current comprising a source ofalternating current, the frequency of which is high in comparisonwith'the frequency of the variationsin the current to be amplified, anelectromagnetic device comprising a pair of windings connected in shuntto each other and included in the circuit of the high frequency current,a pair of parallel cores magnetized by said windings in such a. way thatthe flux produced therein flows in opposite directions in said cores, acontrolling winding surrounding said high frequency windings so as toproduce a flux in the same direction through both of said cores, meansfor supplying the variable current which is to be ampllfied to saidcontrolling winding,

and,condensers in a closed circuit formed I by said high frequencywindings of such capacity that the circuit offers a low impedance tohigh frequency currents but a high impedance to currents of the freuency of the current which is to be amplifie 14. The method ofcontrolling the flow in an electric circuit of a high frequencyalternating current which consists in cansing the current to flowthrough a pair of circuit supplied with energy from sa-id'electriccircuit in such a way that it reaches a state of resonance when theoutput of the main circuit is a minimum.

16. The method of controlling the flow in an electric circuit of a highfre quency 'al- 'ternating current which consists 1n continuouslyvarying the inductance of an auxiliary circuit by varying the magneticpermeability of the circuit and neutralizing part of the inductance byelectrostatic capacity so that theauxiliary circuit reaches a state ofresonance when the output of the main circuit is a minimum.

. 17. The method of controlling the flow in an electric circuit of ahigh frequency alternating current which comprises vary ing aninductance included 'in two auxiliary circuits associated with the maincircuit and neutralizing a part of the inductance by electrostaticcapacities in both of the auxiliary circuits in such a way that one ofthe auxiliary circuits reaches a state of resonance when the output ofthe main circuit is a minimum and the other reaches a state of resonancewhen the output of the main circuit is a maximum.

18. The method of controlling the flow in an electric circuit of a highfrequency alternating current which comprises tuning difierentially twoauxiliarytcircuits associated with the main circuit in such a way thatone of the auxiliary circuits reaches a. state of resonance when theoutput of the main circuit is a minimum and the other reaches astate ofresonance when the output of the main circuit is a maximum.

19. The combination in a wireless signaling system of an antenna circuitsup liedbya source of high frequency alternating current, a reactanceand a variable condenser connected in-shunt to saidhigh frequency sourceand to said antenna circuit, a second circuit including a magnetizingwinding inductively related to sald reactance in such a manner that theimpedance of the antenna circuit may be varied by varying the currentflowing in said magnetizing winding,

and means for causing a variable current to flow in said second circuit.

20. Means for controlling the flow of a radio frequency alternatingcurrent in an electric circuit comprising a pair of magnetic cores, apair of windings for magnetizing said cores which are connected in shuntto each other and supplied with current from said .circuit, means forproducing a low frequency controlling flux in said cores in such a waythat at any given instant the'controlling flux and the radio frequencyflux are in the same direction in one coreand in opposite directions inthe other core, a condenser in series with said windings forneutralizing a part of the voltage of said windings and condensers inthe closed circuit formed byv said windings of such capacity that thecircuit offers a low impedance to radio frequency currents but a highimped- ,ance to currents of the frequency of the controlling flux. i

21. Means for controlling the flow of a radio frequency alternatingcurrent in an electric circuit comprising a pair of magnetic cores, apair of windings magnetizing said cores which are connected in shunt toeach other and supplied with current from said circuit, means forproducing a low frequency controlling flux in said cores in 811011 a waythat at any given instant the controlling fiux and the radio frequencyflux are in the same direction in one core and in opposlte directions inthe other core, and a condenser in series with said windings forneutralizing part of the voltage of said windings.

2?. Means for controlling the flow of a rad1o frequency alternatingcurrent in an electric circuit comprising a 'pair of magnetic cores, apair of windings for magnetizing said cores which are connected in shuntto each other and supplied with current from said circuit, means forroducing a low frequency controlling flux 1n said cores in such a waythat at any given instant the controlling flux and the radio frequencyflux are in the same direction in one core and in opposite directions inthe other core, and condensers in' the closed circuit formed by saidwindings of such capacity that the circuit offers a low impedance tohigh frequency currents but a high impedance to Eurrents of thefrequency of the controlling 23. Means for controlling the flow of aradio frequency alternating current in an electric circuit comprising apair of magnetic cores, a pair of windings for magnetiz= ing said coreswhich are connected in shunt to each other and supplied with currentfrom said circuit, means for roducing a low frequency controlling flux1n said cores in such a way that at any given instant the I controllingflux and the radio frequency flux I windings, the closed circuit formedby said windings being'so arranged that it offers a low impedance to theradio frequency cur- I rents and a high impedance to currents of thecontrolling flux.

the frequency of the controlling flux.

24.'Means for amplifying a variable current comprising a sourceoff-alternating current the frequency of which is high in comparisonwith the frequency-of the variations in the current to be amplified apair of magnetic cores, apair of windings for magnetizing said coreswhich are connected in shunt with each other and supplied from saidsource of alternating current, means for producing by the'current to beamplified a flux in said cores in such away that at any given instant,the flux produced by the variable current and the high frequency fluxare in the same direction in one core and in opposite directions in theother core, and condensers in the closed circuit formed by said windingsof such capacity that the circuit offers a low impedance to high,frequency currents but a high impedance-to the currents of the frequencyof the current which is to be amplified.

25. The combination in a Wireless signaling system of an antenna, asource of radio frequency alternating current connected to said antenna,a pair of magnetic cores, a pair of windings for magnetizing said coreswhich are connected in shunt to each other and included in a shuntcircuit to the antenna circuit, means for producing a low frequencycontrolling flux in said cores in such away that at any given instantthe controlling flux and the radio frequency flux are in the samedirection in one core and in opposite directions in the other core,means for neutralizing a part of the voltage of said windings andcondensers in the closed circuit formed by saidiwindings of such ca,-pacity that the circuit offers a low impedance to radio frequencycurrents but a high impedance to currents of the frequency of ciatedtherewitlia secondary circuit which I "comprises a variable inductanceand a series 26. Means for controlling the flow of radio frequencycurrent in an electric circuit comprising a variable inductance having ashunt and a series capacity associated therewith, the capacities beingso chosen that at one value of the inductanceit is neutralized by theseries capacity at a desired frequency and at a second value of theinductance it isneutralized by the shunt capacity at the same frequency,and means for varying the inductance between said two values.

27. Means for controlling the flow of radio frequency current in anelectric circuit comprising a variable inductance having a shunt and aseries capacity associated therewith, the capacities being so chosenthat at one value of the inductance it is neutralized by theseriescapacity at a desired frequency and at a second value of the inductanceit is neutralized by the shunt capacity at the same frequency.

I 28. A radio signaling. system comprising an antenna which is resonantto a desired frequency supplied by a. source of radio frequencycontinuous current and having assocapacity and a third circuit whichcomprises the variable? inductance and a shunt capacity, the secondcircuit being so proportioned that it becomes resonant to the desiredfrequency at one value of the variable inductance and the third circuitbeingv so proportioned that it becomes resonantto the desired frequencyat a second value of the variable inductance.

29. The method of operating a radiating antenna system which comprises avariable inductance which is common to two oscillating circuits whichconsists in varying the value of said inductance in such a way thatunder one condition of operation one of the oscillating circuits becomesresonant to a desired frequency and under another condition of operatlonthe other oscillating circuit becomes resonant to the desired frequency.

In witness whereof I have set my hand .this 20th January, 1916.

. ERNST F. w. ALEXANDERSON.

